travis: fix CFLAGS assignment error and add -Werror only when compiling on Linux...

travis: fix CFLAGS assignment error and add -Werror only when compiling on Linux for Linux

CFLAGS may not resolve to anything and the build-check.sh script would
just fail.

Switch to EXTRA_FLAGS and use proper substitution so that the script won't
error out when EXTRA_FLAGS is uninitialized.

At the same time ensure that such flag is set only when compiling *for*
linux (we use linux to also compile *for* windows)

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20191110201718.31032-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19109.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>